Hi,
We faced exactly the same problem! I'm quite large on the lower end of my back, but my lovely stoker is rather small. We use exactly this bar, 47 cm version. The extra width won't cause any problem for the stoker, as long as you keep her bar slightly closer to the saddle than on her solo bike. Anyway, on most tandems the stoker have a more cramped position, so the extra width is not an issue.
Alain